A simulcast, or simultaneous broadcast, network is a well known wireless communication system. Such systems are described, for example, in U.S. Pat. No. 6,266,536, herein incorporated by reference. Briefly, a simulcast system is a mobile radio system architecture in which two or more transmitters operate on a single radio frequency over a common area and transmit the same information. Simulcast provides some significant advantages, including wide-area communications with a limited number of channels without the use of a multisite switch. In addition, a simulcast system provides more efficient use of channels in situations where groups operate in multiple locations. Furthermore, simulcast systems offer seamless roaming within the total simulcast coverage area, provide efficient coverage in areas with difficult terrain, and provide improved in-building coverage in some cases due to the multiple transmitter concept.
Nevertheless, simulcast systems are faced with a number of performance issues, especially when digital voice or digital data are transmitted. These issues are addressed with reference to FIG. 5, which shows a simulcast system 510 having only two transmitters 524a and 524b. Both transmitters 524a and 524b are connected to a central control point 523, or master base station, which utilizes special circuitry to transmit a signal to each transmitter 524a and 524b for simultaneous broadcast of the signal in a cell 522a and 522b associated with each transmitter 524a and 524b, respectively, using the same radio frequency (RF). Each transmitter 524a and 524b is connected to the control point 523 via a dedicated, phasestable microwave or optic fiber backbone system.
Typically, there is a delay introduced by the control point 523 in the sending of the signal to the various transmitters 524a and 524b depending upon the distance between the control point 523 and the transmitters 524a and 524b. For example, if a first transmitter 524a is 10 kilometers away from the control point 523, while a second transmitter 524b is 520 kilometers away from the control point 523, the control point 523 will delay sending the signal to the first transmitter 524a, so that the signal will arrive at both transmitters at the exact same time. This difference in transmission times is generally referred to as the timing differential.
Between the transmitters 524a, 524b is a mobile unit 520 is the physical equipment, e.g., a car-mounted mobile radio or other portable radio, used by mobile subscribers to communicate with the mobile radio network 510, each other, and users outside the subscribed network, both wireline and wireless. Theoretically, if the mobile unit is located exactly between the two transmitters 524a and 524b, the signal transmitted from each of the transmitters 524a and 524b would be received by the mobile unit 520 at exactly the same time—i.e., digital bits received by both transmitters 524a and 524b would line up exactly. As the mobile unit 520 moves towards the edge of the overlap zone 525, the mobile unit 520 captures the transmission from one of the transmitters, such as transmitter 524b. This “capture zone” can be defined as the area in which the carrier signal (signal strength) of the closer transmitter 524b exceeds the signal strength of the farther transmitter 524a by at least approximately 10 decibels (dB).
However, as the mobile unit 520 moves through the overlap zone 525 from one of the transmitters 524a towards the other of the transmitters 524b, the interference increases. This is due to the fact that a bit transmitted from the closer transmitter 524b would be received by the mobile unit 520 at an earlier time than the same bit would be received by the mobile unit 520 from the farther transmitter 524a. If this time difference (hereinafter referred to as the delay spread) becomes too large, the symbols begin to interfere with each other, and the mobile unit 520 may demodulate a symbol in error. The symbol errors caused by this self-imposed interference manifest themselves as problems such as limited access to the system, retransmissions of the signal, loss of audio and/or loss of data.
Some systems are tolerant of time delay. For example, in the Enhanced Digital Access Communication System (EDACS®) (M/A Com, Lowell Mass.), transmitter site overlap design parameters allow approximately 30-40 μsec of delay spread with capture ratio ranges of 8-12 dB. The system is theoretically designed so that the mobile unit 520 can receive the signal without significant error. In practice, however, most simulcast systems have some overlap regions in which the overlap design parameters are exceeded and the system coverage is severely degraded or unusable. Additionally, in some simulcast systems, more than two transmitters overlap, which can exacerbate this problem.
Furthermore, this overlap problem is more severe for higher data rates because the ratio of the size of the overlap zone to the capture zone increases. In other words, the probability that a delayed symbol will be demodulated causing a symbol error increases as the clock speed increases. For example, as the bit rate increases from 9.6 kb/s and to 16 kb/s, the theoretical time delay tolerance drops from 50 μsec to 30 μsec, respectively. Furthermore, in practice, the real time delay tolerance is likely to be lower for the reasons mentioned above. Thus, for digital radio transmissions such as control channel, digital voice, or data, a simulcast system almost always provides non-uniform coverage.
Therefore, some kind of equalization of the signals is required to compensate for this time delay. Equalization is a known concept in simulcast networks and involves adjusting the magnitude and phase of received signals using complex channel coefficients to make the signals from different sites essentially equal in magnitude and phase. This concept is considered in greater detail in Equalization—Digital Communications Digital Communications 4th edition 2001 Chapter 11 by John G. Proakis. One well-known approach for equalization is covered by GSM. In GSM, equalization is perforated at the receiver with the help of the training sequences transmitted as part of the midamble in every time slot. The type of equalizer used for GSM is not specified and is left up to the manufacturer as to the method of implementation. Although this standard relates to a high-speed communication system (270 kb/s), it is limited in the time delay for which it can compensate—about 15 μsec. Applicant has determined, however, that such a limitation renders this approach inadequate for typical simulcast networks in which time delays of up to 100 μsec can be expected.
Another possible equalization approach for managing long time delays in a high speed simulcast network involves determining the actual distance between the transmitting sites and using this information to calculate the expected time delay. To this end, the transmitting sites would be outfitted with global positioning devices to provide location information, which would be encoded in the transmitted signal. This approach, although viable, would add complexity and expense to the system.
Therefore, there is a need for equalizing signals in a high speed simulcast network to compensate for large time delay range, while avoiding complex and costly location-determining devices. The present invention fulfills this need among others.